It is known that if access is available to multiple points in a two dimensional array of delay lines supplied with constant amplitude pulses obtained from threshold detection of a video signal then the output from an AND gate supplied with signals from selected ones of the delay lines will represent an eroded or reduced in size representation of the original feature from which the video signal pulses were derived. Conversely the output of an OR gate supplied with the same signals will be enlarged or dilated version of the signals from the original feature.
It is also well known that the same end result can be obtained using two one dimensional operations in sequence thus reducing the hardware required. Thus a series of short delay lines may be employed to obtain access to the signal corresponding to points at small intervals along a line scan and the outputs from the short delay lines can be supplied to an AND gate the output of which can then be supplied to the input of a series connected line of longer duration delay lines each representing one line scan period and the input and outputs of these lines supplied as the inputs to a further AND gate the output of which will be signals representing an eroded version of the original feature.
Also as is well known it is possible to compare signal values in an AND gate derived from points along a line which is not parallel or perpendicular to the direction of line scan but which lies at an angle to the scan by delaying the signals which are to be compared by a time interval not equal to the line scan period but to the line scan period plus or minus a small increment of time depending on how acute the inclined line is to be to the direction of line scan.
In exactly the same way as previously mentioned with regard to operation parallel and subsequently perpendicular to the direction of line scan, so the operation of comparison of points along lines which are inclined to the line scan direction and are not perpendicular thereto can be performed subsequent to the first two operations by supplying the output of the second AND gate previously referred to to the input of a series of appropriate duration delay lines the outputs of which are supplied to the inputs of a further AND gate. The output of this AND gate will then be signals representative of a further erosion of the original feature shape this time in accordance with the direction of the inclined line.
Further erosion on other inclined lines can be performed in similar ways using subsequent operators.
Such techniques are acceptable provided small amendments only are required to the features but the systems necessary and the hardware required become impossibly cumbersome if a large number of line scans of erosion are required.
One technique to reduce the hardware is to sample every nth line and hold the data in that line static for the intervening (n-1) lines. Although this reduces hardware it creates harmonic responses when the features are not solid and incorrect results are obtained from the analysis.